Under-display ultrasonic fingerprint sensors for foldable displays

ABSTRACT

An apparatus may include an ultrasonic sensor stack, a foldable display stack and a transmission enhancement layer. The foldable display stack may include a display stiffener and display stack layers. The display stack layers may form one or more display stack resonators configured to enhance ultrasonic waves transmitted by the ultrasonic sensor stack in a first ultrasonic frequency range. In some implementations, a transmission enhancement resonator may include the display stiffener and the transmission enhancement layer. In some examples, the transmission enhancement resonator may include at least a portion of the ultrasonic sensor stack. The transmission enhancement resonator may be configured to enhance the ultrasonic waves transmitted by the ultrasonic sensor stack in the first ultrasonic frequency range.

TECHNICAL FIELD

This disclosure relates generally to sensor devices and related methods,including but not limited to ultrasonic sensor systems and methods forusing such systems.

DESCRIPTION OF THE RELATED TECHNOLOGY

Biometric authentication can be an important feature for controllingaccess to devices, etc. Many existing products include some type ofbiometric authentication. Although some existing biometricauthentication technologies provide satisfactory performance, improvedmethods and devices would be desirable.

SUMMARY

The systems, methods and devices of the disclosure each have severalinnovative aspects, no single one of which is solely responsible for thedesirable attributes disclosed herein.

One innovative aspect of the subject matter described in this disclosuremay be implemented in an apparatus. The apparatus may include anultrasonic sensor stack, a foldable display stack and a transmissionenhancement layer. The foldable display stack may, in some examples,include a display stiffener and display stack layers configured to causeultrasonic waves transmitted by the ultrasonic sensor stack to have oneor more display stack-induced local amplitude maxima in a firstultrasonic frequency range. In some examples, the display stiffener andthe transmission enhancement layer may form at least part of atransmission enhancement resonator. The transmission enhancementresonator may, in some implementations, be configured to cause theultrasonic waves transmitted by the ultrasonic sensor stack to have atransmission enhancement resonator-induced local amplitude maximum inthe first ultrasonic frequency range.

According to some implementations, the apparatus may be a mobile device.The mobile device may include the ultrasonic sensor stack, the foldabledisplay stack and the transmission enhancement layer.

In some examples, the transmission enhancement resonator-induced localamplitude maximum may correspond to a frequency in the range from 5 MHzto 15 MHz. In other examples, the transmission enhancementresonator-induced local amplitude maximum may correspond to a frequencyin the range from 1 MHz to 20 MHz. In some examples, the transmissionenhancement resonator may have a thickness corresponding to a multipleof a half wavelength of a shear wave or a longitudinal wave having afrequency in the first ultrasonic frequency range.

According to some examples, at least some of the display stack layersmay include (and/or may form) one or more display stack resonators. Insome such examples, the one or more display stack resonators may beconfigured to cause the one or more display stack-induced localamplitude maxima. In some examples, the one or more display stackresonators may include a first resonator bounded by the displaystiffener and a glass layer of the foldable display stack. According tosome such examples, the first resonator may include a plurality oflayers of an organic light-emitting diode display. In some examples, theone or more display stack resonators may include a second resonatorbounded by the glass layer and an outer surface of the foldable displaystack.

In some examples, the transmission enhancement layer may reside betweenthe ultrasonic sensor stack and the display stiffener. According to someexamples, the transmission enhancement layer may have a thickness ofless than a quarter wavelength of a shear wave or a longitudinal wavehaving a frequency in the first ultrasonic frequency range. In someimplementations, the transmission enhancement layer may be, or mayinclude, one or more of an aluminum layer having an aluminum layerthickness in a range from 50 microns to 100 microns, a copper layerhaving a copper layer thickness in a range from 25 microns to 50 micronsor a stainless steel layer having a stainless steel layer thickness inthe range from 25 microns to 50 microns. According to some examples, thetransmission enhancement layer may include at least a portion of theultrasonic sensor stack.

According to some implementations, the apparatus may include a firstadhesive layer residing between the transmission enhancement layer andthe display stiffener. In some implementations, the apparatus mayinclude a second adhesive layer residing between the transmissionenhancement layer and the ultrasonic sensor stack. According to someexamples, the first adhesive layer and/or the second adhesive layer mayhave a thickness in the range from 3 microns to 10 microns.

In some examples, the ultrasonic sensor stack may include a thin-filmtransistor (TFT) substrate. According to some such examples, thetransmission enhancement resonator may include the TFT substrate. Insome instances, the TFT substrate may have a thickness of less than aquarter wavelength of a shear wave or a longitudinal wave having afrequency in the first ultrasonic frequency range. In some examples, theTFT substrate may have a thickness in a range from 50 microns to 200microns. According to some examples, the TFT substrate may be made of,or may include, glass.

According to some implementations, the TFT substrate may have a firstacoustic impedance value. In some such examples, the display stiffenermay have a second acoustic impedance value that is greater than thefirst acoustic impedance value. In some examples, the transmissionenhancement layer may have a third acoustic impedance value that isgreater than the first acoustic impedance value.

According to some examples, the display stiffener may be, or mayinclude, one or more of a metal layer or a non-metal layer having anacoustic impedance of 10 MRayls or more. In some examples, the displaystiffener may have a thickness in a range from 30 microns to 300microns. According to some examples, the display stiffener may have athickness corresponding to a multiple of a half wavelength of a shearwave or a longitudinal wave having a frequency in a second ultrasonicfrequency range that is higher than the first ultrasonic frequencyrange.

In some implementations, the apparatus may include a control system. Thecontrol system may include one or more general purpose single- ormulti-chip processors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs) or other programmable logic devices, discrete gates ortransistor logic, discrete hardware components, or combinations thereof.According to some examples, the control system may be configured tocause the ultrasonic sensor stack to transmit first ultrasonic waves inthe first ultrasonic frequency range and to perform an authenticationprocess based, at least in part, on ultrasonic sensor signalscorresponding to reflections of the first ultrasonic waves.

Other innovative aspects of the subject matter described in thisdisclosure may be implemented in a method. In some examples, the methodmay involve controlling, via a control system, an ultrasonic transceiverlayer of an ultrasonic sensor system to transmit ultrasonic wavesthrough at least a first resonator configured for causing a first localmaximum of ultrasonic wave transmission in a first ultrasonic frequencyrange and a second resonator configured for causing a second localmaximum of ultrasonic wave transmission in the first ultrasonicfrequency range. In some examples, the first resonator may include adisplay stiffener layer and a transmission enhancement layer. In someinstances, the second resonator may include one or more display stacklayers.

In some examples, the method may involve receiving, by the controlsystem and from the ultrasonic sensor system, ultrasonic sensor signalscorresponding to reflections of transmitted ultrasonic waves from aportion of a target object positioned on an outer surface of anapparatus that includes the ultrasonic sensor system. According to someexamples, the method may involve performing, by the control system, anauthentication process based, at least in part, on the ultrasonic sensorsignals.

According to some implementations, the local maximum of ultrasonic wavetransmission may correspond to a frequency in a range from 5 MHz to 15MHz. However, in other implementations, the local maximum of ultrasonicwave transmission may correspond to a frequency in a range from 1 MHz to20 MHz.

In some examples, the authentication process may involve extractingtarget object features from the ultrasonic sensor signals. The targetobject features may, for example include fingerprint features and/orsub-epidermal features. In some examples, the method may involvecontrolling access to the apparatus based, at least in part, on theauthentication process.

Some or all of the operations, functions and/or methods described hereinmay be performed by one or more devices according to instructions (e.g.,software) stored on one or more non-transitory media. Suchnon-transitory media may include memory devices such as those describedherein, including but not limited to random access memory (RAM) devices,read-only memory (ROM) devices, etc. Accordingly, some innovativeaspects of the subject matter described in this disclosure can beimplemented in one or more non-transitory media having software storedthereon.

For example, the software may include instructions for controlling oneor more devices to perform a method. According to some examples, themethod may involve controlling, via a control system, an ultrasonictransceiver layer of an ultrasonic sensor system to transmit ultrasonicwaves through at least a first resonator configured for causing a firstlocal maximum of ultrasonic wave transmission in a first ultrasonicfrequency range and a second resonator configured for causing a secondlocal maximum of ultrasonic wave transmission in the first ultrasonicfrequency range. In some examples, the first resonator may include adisplay stiffener layer and a transmission enhancement layer. In someinstances, the second resonator may include one or more display stacklayers.

In some examples, the method may involve receiving, by the controlsystem and from the ultrasonic sensor system, ultrasonic sensor signalscorresponding to reflections of transmitted ultrasonic waves from aportion of a target object positioned on an outer surface of anapparatus that includes the ultrasonic sensor system. According to someexamples, the method may involve performing, by the control system, anauthentication process based, at least in part, on the ultrasonic sensorsignals.

According to some implementations, the local maximum of ultrasonic wavetransmission may correspond to a frequency in a range from 5 MHz to 15MHz. However, in other implementations, the local maximum of ultrasonicwave transmission may correspond to a frequency in a range from 1 MHz to20 MHz.

In some examples, the authentication process may involve extractingtarget object features from the ultrasonic sensor signals. The targetobject features may, for example include fingerprint features and/orsub-epidermal features. In some examples, the method may involvecontrolling access to the apparatus based, at least in part, on theauthentication process.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of one or more implementations of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings, and the claims. Note thatthe relative dimensions of the following figures may not be drawn toscale. Like reference numbers and designations in the various drawingsindicate like elements.

FIG. 1 is a block diagram that shows example components of an apparatusaccording to some disclosed implementations.

FIG. 2 shows examples of a foldable display stack and an ultrasonicsensor stack.

FIG. 3 shows examples of ultrasonic transmission versus ultrasonicfrequency graphs corresponding to examples of the resonators shown inFIG. 2 .

FIG. 4 shows examples of a foldable display stack and an ultrasonicsensor stack according to some disclosed implementations.

FIG. 5 shows examples of ultrasonic transmission versus ultrasonicfrequency graphs corresponding to examples of the resonators shown inFIG. 4 .

FIG. 6 shows examples of a foldable display stack and an ultrasonicsensor stack.

FIG. 7 shows example components of an apparatus according to somedisclosed implementations.

FIG. 8 is a flow diagram that provides examples of operations accordingto some disclosed methods.

FIG. 9 representationally depicts aspects of a 4×4 pixel array of sensorpixels for an ultrasonic sensor system.

DETAILED DESCRIPTION

The following description is directed to certain implementations for thepurposes of describing the innovative aspects of this disclosure.However, a person having ordinary skill in the art will readilyrecognize that the teachings herein may be applied in a multitude ofdifferent ways. The described implementations may be implemented in anydevice, apparatus, or system that includes a biometric system asdisclosed herein. In addition, it is contemplated that the describedimplementations may be included in or associated with a variety ofelectronic devices such as, but not limited to: mobile telephones,multimedia Internet enabled cellular telephones, mobile televisionreceivers, wireless devices, smartphones, smart cards, wearable devicessuch as bracelets, armbands, wristbands, rings, headbands, patches,etc., Bluetooth® devices, personal data assistants (PDAs), wirelesselectronic mail receivers, hand-held or portable computers, netbooks,notebooks, smartbooks, tablets, printers, copiers, scanners, facsimiledevices, global positioning system (GPS) receivers/navigators, cameras,digital media players (such as MP3 players), camcorders, game consoles,wrist watches, clocks, calculators, television monitors, flat paneldisplays, electronic reading devices (e.g., e-readers), mobile healthdevices, computer monitors, auto displays (including odometer andspeedometer displays, etc.), cockpit controls and/or displays, cameraview displays (such as the display of a rear view camera in a vehicle),electronic photographs, electronic billboards or signs, projectors,architectural structures, microwaves, refrigerators, stereo systems,cassette recorders or players, DVD players, CD players, VCRs, radios,portable memory chips, washers, dryers, washer/dryers, parking meters,packaging (such as in electromechanical systems (EMS) applicationsincluding microelectromechanical systems (MEMS) applications, as well asnon-EMS applications), aesthetic structures (such as display of imageson a piece of jewelry or clothing) and a variety of EMS devices. Theteachings herein also may be used in applications such as, but notlimited to, electronic switching devices, radio frequency filters,sensors, accelerometers, gyroscopes, motion-sensing devices,magnetometers, inertial components for consumer electronics, parts ofconsumer electronics products, steering wheels or other automobileparts, varactors, liquid crystal devices, electrophoretic devices, driveschemes, manufacturing processes and electronic test equipment. Thus,the teachings are not intended to be limited to the implementationsdepicted solely in the Figures, but instead have wide applicability aswill be readily apparent to one having ordinary skill in the art.

It is challenging to design an under-display ultrasonic sensor systemthat provides acceptable performance. However, the present assignee hassuccessfully designed under-display ultrasonic sensor systems that arewidely deployed in cell phones and other display devices.

Designing an under-display ultrasonic sensor system for foldable displaydevices presents additional challenges. A foldable display includes adisplay stiffener, which helps to maintain the physical integrity of thedisplay stack when the foldable display is folded and unfolded. Thedisplay stiffener is usually made from metal having a thickness in therange of approximately 30 microns to 300 microns, though displaystiffeners also may be formed of other material(s) having a relativelyhigh modulus of elasticity and may, in some instances, have otherthicknesses. The present inventors have found that display stiffenersseverely attenuate ultrasonic waves in frequency ranges that aresuitable for ultrasonic fingerprint sensors.

Some disclosed foldable display devices include an ultrasonic sensorstack, a transmission enhancement layer and a foldable display stackhaving a display stiffener. In some examples, the display stiffener, thetransmission enhancement layer and at least a portion of the ultrasonicsensor stack form a transmission enhancement resonator that isconfigured to enhance the ultrasonic waves transmitted by the ultrasonicsensor stack in an ultrasonic frequency range that is suitable forultrasonic fingerprint sensors.

Particular implementations of the subject matter described in thisdisclosure may be implemented to realize one or more of the followingpotential advantages. Some disclosed foldable display devices haveenhanced transmission of ultrasonic waves in an ultrasonic frequencyrange that is suitable for ultrasonic fingerprint sensors, as comparedto the transmission of ultrasonic waves of foldable display devices thatlack some version of the disclosed transmission enhancement layer.Implementations wherein the transmission enhancement layer residesbetween an ultrasonic sensor stack and a display stiffener may bemanufactured without modifying the existing foldable display stack,thereby avoiding additional manufacturing cost and time.

FIG. 1 is a block diagram that shows example components of an apparatusaccording to some disclosed implementations. As with other disclosedimplementations, the numbers, types and arrangements of elements shownin FIG. 1 are merely presented by way of example. Although not shown inFIG. 1 , the apparatus 100 may include other components, such as a cover(which may be, or may include, a cover glass), one or more adhesivelayers, one or more electrode layers, etc. Some examples are describedbelow. In some implementations, the apparatus 100 may be a mobile devicethat includes the elements shown in FIG. 1 .

According to this example, the apparatus 100 includes an ultrasonicsensor stack 105. In some examples, the ultrasonic sensor stack 105includes an ultrasonic transceiver layer 101 and a thin-film transistor(TFT) layer 102. In some such examples, the ultrasonic transceiver layer101 may be configured to function as both an ultrasonic transmitter andan ultrasonic receiver. According to some implementations, theultrasonic transceiver layer 101 may be a single piezoelectric layer,whereas in other implementations the ultrasonic transceiver layer 101may be a multilayer piezoelectric structure, or an array of suchstructures.

For example, in some implementations, the ultrasonic transceiver layer101 may include a piezoelectric layer, such as a layer of PVDF polymeror a layer of PVDF-TrFE copolymer. In some implementations, otherpiezoelectric materials may be used in the ultrasonic transceiver layer101, such as aluminum nitride (AlN) or lead zirconate titanate (PZT).Some alternative implementations may include separate ultrasonictransmitter and ultrasonic receiver layers.

The ultrasonic transceiver layer 101 may, in some alternative examples,include an array of ultrasonic transducer elements, such as an array ofpiezoelectric micromachined ultrasonic transducers (PMUTs), an array ofcapacitive micromachined ultrasonic transducers (CMUTs), etc. In somesuch examples, a piezoelectric receiver layer, PMUT elements in asingle-layer array of PMUTs, or CMUT elements in a single-layer array ofCMUTs, may be used as ultrasonic transmitters as well as ultrasonicreceivers.

The TFT layer 102 may be a type of metal-oxide-semiconductorfield-effect transistor (MOSFET) made by depositing thin films of anactive semiconductor layer as well as a dielectric layer and metalliccontacts over a TFT substrate. In some examples, the TFT substrate maybe a non-conductive material such as glass. According to someimplementations, the TFT layer 102 may have a thickness that is in therange of 50 microns to 400 microns.

In this implementation, the apparatus includes a foldable display stack111. According to this example, the foldable display stack 111 includesa display stiffener 113 and display stack layers 115. The display stacklayers 115 may, in some examples, include layers of a light-emittingdiode (LED) display, such as an organic light-emitting diode (OLED)display. Some examples of display stack layers 115 are provided in thisdisclosure.

In this example, the display stack layers 115 form one or more displaystack resonators. The display stack resonator(s) may, in some examples,be configured to enhance ultrasonic waves transmitted by the ultrasonicsensor stack in a first ultrasonic frequency range. In some examples,the one or more display stack resonators may include a first resonatorbounded by the display stiffener 113 and a glass layer of the displaystack layers 115. In some such examples, the first resonator may includea plurality of layers of an organic light-emitting diode display. Insome examples, the one or more display stack resonators may include asecond resonator bounded by the glass layer and an outer surface of thefoldable display stack.

In some examples, the display stiffener 113 may have a relatively highacoustic impedance, e.g., an acoustic impedance of 10 MRayls or more. Insome implementations, the display stiffener 113 may be, or may include,a metal layer (e.g., a stainless steel layer having an acousticimpedance of approximately 47 MRayls). However, in other implementationsthe display stiffener 113 may be, or may include, one or more othermetals, or non-metal material having a relatively high modulus ofelasticity. According to some examples, the display stiffener 113 mayhave a thickness in the range of 30 microns to 300 microns. According tosome examples, the display stiffener 113 may have a thicknesscorresponding to a multiple of a half wavelength of a shear wave or alongitudinal wave having a frequency in a second ultrasonic frequencyrange that is higher than the first ultrasonic frequency range. However,in some instances the display stiffener 113 may not be, or may notinclude, a material having a high acoustic impedance. For example, insome instances the display stiffener 113 may be, or may include, aplastic layer such as a polycarbonate layer. In some such instances, thedisclosed transmission enhancement layer 103 may not be beneficial.

According to this example, the apparatus 100 includes a transmissionenhancement layer 103. In some examples, the transmission enhancementlayer 103 may be, or may include, an aluminum layer having a thicknessin a range from 50 microns to 100 microns, a copper layer having athickness in a range from 25 microns to 50 microns or a stainless steellayer having a thickness in the range from 25 microns to 50 microns.According to some examples, the transmission enhancement layer 103 mayhave a thickness of less than a quarter wavelength of a shear wave or alongitudinal wave having a frequency in the first ultrasonic frequencyrange. In some examples, the transmission enhancement layer 103 mayreside between the ultrasonic sensor stack 105 and the foldable displaystack 111. In some such examples, the transmission enhancement layer 103may reside between the ultrasonic sensor stack 105 and the displaystiffener 113. According to some such examples, the display stiffener113, the transmission enhancement layer 103 and at least a portion ofthe ultrasonic sensor stack 105 (e.g., the TFT substrate of the TFTlayer 102) form a transmission enhancement resonator that is configuredto enhance the ultrasonic waves transmitted by the ultrasonic sensorstack in an ultrasonic frequency range that is suitable for ultrasonicfingerprint sensors. Some examples are described below.

In some examples, the apparatus 100 may include a control system 106.The control system 106 (when present) may include one or more generalpurpose single- or multi-chip processors, digital signal processors(DSPs), application specific integrated circuits (ASICs), fieldprogrammable gate arrays (FPGAs) or other programmable logic devices,discrete gates or transistor logic, discrete hardware components, orcombinations thereof. The control system 106 also may include (and/or beconfigured for communication with) one or more memory devices, such asone or more random access memory (RAM) devices, read-only memory (ROM)devices, etc. Accordingly, the apparatus 100 may have a memory systemthat includes one or more memory devices, though the memory system isnot shown in FIG. 1 . The control system 106 may be capable of receivingand processing data from the ultrasonic transceiver layer 101 and/orfrom an array of sensor pixels, e.g., as described below. In someimplementations, functionality of the control system 106 may bepartitioned between one or more controllers or processors, such as adedicated sensor controller and an applications processor of a mobiledevice.

Some implementations of the apparatus 100 may include an interfacesystem 107. In some examples, the interface system may include awireless interface system. In some implementations, the interface systemmay include a user interface system, one or more network interfaces, oneor more interfaces between the control system 106 and a memory systemand/or one or more interfaces between the control system 106 and one ormore external device interfaces (e.g., ports or applicationsprocessors).

The interface system 107 may be configured to provide communication(which may include wired or wireless communication, such as electricalcommunication, radio communication, etc.) between components of theapparatus 100. In some such examples, the interface system 107 may beconfigured to provide communication between the control system 106 andthe ultrasonic receiver layer 101, to provide communication between thecontrol system 106 and one or more of the display stack layers 115and/or to provide communication between the control system 106 and anarray of sensor pixels. According to some such examples, a portion ofthe interface system 107 may couple at least a portion of the controlsystem 106 to the ultrasonic receiver layer 101 and/or an array ofsensor pixels, e.g., via electrically conducting material.

According to some examples, the interface system 107 may be configuredto provide communication between the apparatus 100 and other devicesand/or human beings. In some such examples, the interface system 107 mayinclude one or more user interfaces. The interface system 107 may, insome examples, include one or more network interfaces and/or one or moreexternal device interfaces (such as one or more universal serial bus(USB) interfaces). In some implementations, the apparatus 100 mayinclude a memory system. The interface system 107 may, in some examples,include at least one interface between the control system 106 and amemory system.

The apparatus 100 may be used in a variety of different contexts, manyexamples of which are disclosed herein. For example, in someimplementations a mobile device, such as a cell phone, a smart phone, atablet, a laptop (e.g., a laptop touchpad), etc., may include at least aportion of the apparatus 100. In some implementations, a wearable devicemay include at least a portion of the apparatus 100. The wearable devicemay, for example, be a watch, a bracelet, an armband, a wristband, aring, a headband or a patch. In some implementations, the control system106 may reside in more than one device. For example, a portion of thecontrol system 106 may reside in a wearable device and another portionof the control system 106 may reside in another device, such as a mobiledevice (e.g., a smartphone or a tablet computer) and/or a server. Theinterface system 107 also may, in some such examples, reside in morethan one device.

FIG. 2 shows examples of a foldable display stack and an ultrasonicsensor stack. The types, number and arrangement of elements shown inFIG. 2 are merely examples. Other examples may include different types,numbers and/or arrangements of elements. Moreover, the elements shown inFIG. 2 are not drawn to scale.

The apparatus 200 is similar to the apparatus 100 shown in FIG. 1 , butdoes not include the transmission enhancement layer 103. The apparatus200 includes instances of the ultrasonic sensor stack 105 and of thefoldable display stack 111 of FIG. 1 . Here, the ultrasonic sensor stack105 is attached to the foldable display stack 111 via an adhesive layer202 a. The adhesive layer 202 a may be, or may include, a thinpressure-sensitive adhesive (PSA). In some instances, the adhesive layer202 a may be in the range of 2 microns to 10 microns.

The ultrasonic sensor stack 105 includes a TFT layer 102, an ultrasonictransceiver layer 101 and an electrode layer 210. The TFT layer 102resides between the ultrasonic transceiver layer 101 and the foldabledisplay stack 111, and the adhesive layer 202 a connects the TFT layer102 to the foldable display stack 111. In some alternative examples, theultrasonic transceiver layer 101 may reside between the TFT layer 102and the foldable display stack 111. The ultrasonic transceiver layer 101may be, or may include, one or more piezoelectric materials, such as apiezoelectric polymer and/or a piezoelectric copolymer. The electrodelayer 210 may be, or may include, a conductive ink (e.g., silver ink).In this instance, the ultrasonic sensor stack 105 includes a passivationlayer 212. The passivation layer 212 may be, or may include, an epoxyfilm.

The foldable display stack 111 includes a display stiffener 113 thatresides between the TFT layer 102 and the other layers of the foldabledisplay stack 111. The display stiffener 113 provides structural supportfor the other layers of the foldable display stack 111. In someexamples, the display stiffener 113 may be, or may include, ahigh-impedance material (in other words, a material having a highacoustic impedance) such as metal.

The foldable display stack 111 includes one or more screen protectorlayers 211, which may include a polyethylene terephthalate (PET) layerin some instances. The foldable display stack 111 includes a glass layer203, a polarizer layer 222, an OLED panel 214 and one or more layers ofprotective film 215. Here, an optically clear adhesive (OCA) layer 204 aconnects the one or more screen protector layers 211 to the glass layer203 and an OCA layer 204 b connects the glass layer 203 to the polarizerlayer 222. A polarizer pressure-sensitive adhesive 213 a connects thepolarizer 222 to the OLED panel 214 and a pressure-sensitive adhesive213 b connects the OLED panel 214 to the one or more layers ofprotective film 215. The polarizer pressure-sensitive adhesive 213 amay, for example, be an optically clear adhesive (OCA).

In this example, the adhesive layer 202 b and layers of the foldabledisplay stack 111 form the resonator 250 a, which is bounded by theglass layer 203 and the display stiffener 113. According to thisexample, the resonator 250 a includes the OCA layer 204 b, the polarizerlayer 222, the polarizer pressure-sensitive adhesive 213 a, the OLEDpanel 214, the pressure-sensitive adhesive 213 b, the one or more layersof protective film 215 and the adhesive layer 202 b. In some examples,the thickness of the resonator 250 a may correspond to a multiple N of ahalf wavelength at a peak frequency of an ultrasonic frequency range ofultrasonic waves transmitted by the ultrasonic sensor stack 105, where Nis an integer greater than or equal to 1. In some such implementations,the resonator 250 a may cause a local maximum within the ultrasonicfrequency range. According to some examples, the local maximum maycorrespond to a frequency in the range from 5 MHz to 15 MHz, or from 1MHz to 20 MHz. According to some such implementations, a frequency rangethat includes the local ultrasonic wave transmission maximum caused bythe resonator 250 a may correspond with a frequency range that includesa local ultrasonic wave transmission maximum caused by the resonator 250c.

According to this example, the foldable display stack 111 also includesthe resonator 250 c, which is formed by the one or more screen protectorlayers 211 and the OCA 204 a, and is bounded by the glass layer 203:here, the glass layer 203 has a higher acoustic impedance than that ofthe one or more screen protector layers 211 or the OCA 204 a. In someexamples, the thickness of the resonator 250 c may correspond to amultiple N of a quarter wavelength at a peak frequency of an ultrasonicfrequency range of ultrasonic waves transmitted by the ultrasonic sensorstack 105, where N is an integer greater than or equal to 1. In somesuch implementations, the resonator 250 c may cause a local maximumwithin the ultrasonic frequency range. According to some examples, thelocal maximum may correspond to a frequency in the range from 5 MHz to15 MHz, or from 1 MHz to 20 MHz. According to some such implementations,a frequency range that includes the local ultrasonic wave transmissionmaximum caused by the resonator 250 a may correspond with a frequencyrange that includes a local ultrasonic wave transmission maximum causedby the resonator 250 c.

In this implementation, the glass layer 203 forms a resonator 250 b.According to some examples, the frequencies for maximum transmission ofultrasonic waves through the resonator 250 b may be outside of (e.g.,below) the frequency range that includes the local maxima caused by theresonators 250 a and 250 c. In some examples, the thickness of theresonator 250 b may be less than a multiple N of a quarter wavelength ata peak frequency of an ultrasonic frequency range of ultrasonic wavestransmitted by the ultrasonic sensor stack 105, where N is an integergreater than or equal to 1.

In this example, the display stiffener 113 forms the resonator 250 d.According to this example, the resonator 250 d causes a very lowtransmission of ultrasonic waves in the frequency range from 5 MHz to 15MHz. The low transmission of ultrasonic waves caused by the displaystiffener 113 represents a significant challenge for the design ofunder-display ultrasonic sensors.

FIG. 3 shows examples of ultrasonic transmission versus ultrasonicfrequency graphs corresponding to examples of the resonators shown inFIG. 2 . In these examples, graph 305 a indicates the ultrasonictransmission amplitudes of the resonator 250 a at various frequencies,graph 305 b indicates the ultrasonic transmission amplitudes of theresonator 250 b at various frequencies, graph 305 c indicates theultrasonic transmission amplitudes of the resonator 250 c at variousfrequencies and graph 305 d indicates the ultrasonic transmissionamplitudes of the resonator 250 d at various frequencies. However, theseultrasonic transmission amplitudes are merely specific examples, basedon the thicknesses and materials used in a particular foldable displaystack 111.

In these examples, the frequency band 301 between frequencies F1 and F2is a desirable frequency band for ultrasonic imaging relating toauthentication. In some examples, the frequency band 301 may be suitablefor fingerprint imaging, whereas in some examples the frequency band 301may be suitable for sub-epidermal imaging. In some instances, thefrequency band 301 may include frequencies in the range from 5 MHz to 15MHz, or in the range from 1 MHz to 20 MHz. In one example, the frequencyband 301 is in the range of 6 MHz to 8 MHz. In another example, thefrequency band 301 is in the range from 11 MHz to 14 MHz.

According to the examples shown in FIG. 3 , the resonators 250 a and 250c cause a local ultrasonic transmission amplitude maximum within theultrasonic frequency band 301. These local amplitude maxima are examplesof what may be referred to herein as “display stack-induced localamplitude maxima.” In these examples, the peak frequency of the localultrasonic transmission maximum caused by the resonator 250 a is lowerthan the peak frequency of the local ultrasonic transmission maximumcaused by the resonator 250 c. In this instance, graph 305 b indicatesno local ultrasonic transmission maximum caused by the resonator 250 bwithin the ultrasonic frequency band 301. However, the level ofultrasonic transmission caused by the resonator 250 b is greater forfrequencies within the ultrasonic frequency band 301 than for higherfrequencies. The terms “frequency band” and “frequency range” are usedsynonymously in this disclosure.

In this example, the resonator 250 d—corresponding to the displaystiffener 113 shown in FIG. 2 —causes a local ultrasonic transmissionmaximum at a peak frequency of F3, within a second ultrasonic frequencyband 307 that spans a higher frequency range (in this example, fromfrequency F4 to frequency F5) than the ultrasonic frequency band 301.However, in this example, the resonator 250 d causes very low levels ofultrasonic transmission within the frequency band 301. Accordingly, thegraph 305 d shows an example of the low levels of ultrasonic wavetransmission within a frequency band suitable for ultrasonic fingerprintsensor imaging that is caused by the display stiffener 113.

Although graphs 305 a-305 d are merely examples corresponding to aparticular foldable display stack configuration, the present inventorshave found it to be generally true that display stiffeners of foldabledisplay stacks often cause a low transmission of ultrasonic waves in afrequency band for suitable for fingerprint imaging (e.g., in the rangefrom 10 MHz to 20 MHz) and/or a frequency band for suitable forsub-epidermal imaging (e.g., in the range from 1 MHz to 10 MHz). Inresponse to this design challenge, the present inventors have developedmethods and devices for improving the transmissions of ultrasonic wavesin one or more frequency bands suitable for fingerprint imaging and/orsub-epidermal imaging. In some disclosed implementations, the displaystiffener, the transmission enhancement layer and at least a portion ofthe ultrasonic sensor stack form a transmission enhancement resonatorthat is configured to enhance the ultrasonic waves transmitted by theultrasonic sensor stack in an ultrasonic frequency range that issuitable for ultrasonic fingerprint sensors.

FIG. 4 shows examples of a foldable display stack and an ultrasonicsensor stack according to some disclosed implementations. As with otherdisclosed implementations, the types, number and arrangement of elementsshown in FIG. 4 are merely examples. Other implementations may includedifferent types, numbers and/or arrangements of elements. Moreover, theelements shown in FIG. 4 are not drawn to scale.

In this example the apparatus 100 a is similar to the apparatus 200shown in FIG. 2 . However, in this implementation the apparatus 100 aincludes an instance of the transmission enhancement layer 103 that isdescribed above with reference to FIG. 1 . According to this example,the apparatus 100 a includes instances of the ultrasonic sensor stack105 and of the foldable display stack 111 of FIG. 1 . In this example,the transmission enhancement layer 103 is attached to the foldabledisplay stack 111 via an adhesive layer 202 a and the ultrasonic sensorstack 105 a is attached to the transmission enhancement layer 103 via anadhesive layer 202 c. In some examples, the adhesive layers 202 a and/or202 c may be, or may include, a thin pressure-sensitive adhesive (PSA).In some instances, the adhesive layers 202 a and/or 202 c may be in therange of 1 micron to 10 microns.

According to this example, the ultrasonic sensor stack 105 a includes aTFT layer 102, an ultrasonic transceiver layer 101 and an electrodelayer 210. According to some implementations, the thickness of the TFTlayer 102 may be in the range of 50 microns to 150 microns, e.g., 50microns, 55 microns, 60 microns, 65 microns, 70 microns, 75 microns, 80microns, 85 microns, 90 microns, 95 microns, 100 microns, 105 microns,110 microns, 115 microns, 120 microns, 125 microns, 130 microns, 135microns, 140 microns, 145 microns or 150 microns. In this example, theTFT layer 102 resides between the ultrasonic transceiver layer 101 andthe foldable display stack 111, and the adhesive layer 202 a connectsthe TFT layer 102 to the foldable display stack 111. In some alternativeimplementations, the ultrasonic transceiver layer 101 may reside betweenthe TFT layer 102 and the foldable display stack 111.

According to some examples, the ultrasonic transceiver layer 101 may be,or may include, one or more piezoelectric materials, such as apiezoelectric polymer and/or a piezoelectric copolymer. In someexamples, the thickness of the ultrasonic transceiver layer 101 may bein the range of 5 microns to 20 microns, e.g., 5 microns, 6 microns, 7microns, 8 microns, 9 microns, 10 microns, 11 microns, 12 microns, 13microns, 14 microns, 15 microns, 16 microns, 17 microns, 18 microns, 19microns or 20 microns.

According to some implementations, the electrode layer 210 may be, ormay include, a conductive ink (e.g., silver ink). However, in otherimplementations the electrode layer 210 may be, or may include, othertypes of conductors, such as copper, gold, etc. In some examples, thethickness of the electrode layer 210 may be in the range of 10 micronsto 30 microns, e.g., 10 microns, 11 microns, 12 microns, 13 microns, 14microns, 15 microns, 16 microns, 17 microns, 18 microns, 19 microns, 20microns, 21 microns, 22 microns, 23 microns, 24 microns, 25 microns, 26microns, 27 microns, 28 microns, 29 microns or 30 microns.

In this instance, the ultrasonic sensor stack 105 a includes apassivation layer 212. According to some examples, the passivation layer212 may be, or may include, an epoxy film. In some examples, thepassivation layer 212 may be in the range of 10 microns to 30 microns,e.g., 10 microns, 11 microns, 12 microns, 13 microns, 14 microns, 15microns, 16 microns, 17 microns, 18 microns, 19 microns, 20 microns, 21microns, 22 microns, 23 microns, 24 microns, 25 microns, 26 microns, 27microns, 28 microns, 29 microns or 30 microns.

In this example, the foldable display stack 111 includes the same layersthat are described above with reference to FIG. 2 . In this instance,the foldable display stack 111 includes a display stiffener 113 thatresides between the TFT layer 102 and the other layers of the foldabledisplay stack 111. The display stiffener 113 provides structural supportfor the other layers of the foldable display stack 111. In someexamples, the display stiffener 113 may be, or may include, ahigh-impedance material such as metal. According to some suchimplementations, the display stiffener 113 may include stainless steel.However, in other implementations the display stiffener 113 may be, ormay include, one or more other metals, or non-metal material having arelatively high modulus of elasticity. In some examples, the displaystiffener 113 may have a thickness in the range of 30 microns to 300microns, e.g., 30 microns, 35 microns, 40 microns, 45 microns, 50microns, 55 microns, 60 microns, 65 microns, 70 microns, 75 microns, 80microns, 85 microns, 90 microns, 95 microns, 100 microns, 105 microns,110 microns, 115 microns, 120 microns, 125 microns, 130 microns, 135microns, 140 microns, 145 microns, 150 microns, 155 microns, 160microns, 165 microns, 170 microns, 175 microns, 180 microns, 185microns, 190 microns, 195 microns, 200 microns, 205 microns, 210microns, 215 microns, 220 microns, 225 microns, 230 microns, 235microns, 240 microns, 245 microns, 250 microns, 255 microns, 260microns, 265 microns, 270 microns, 275 microns, 280 microns, 285microns, 290 microns, 295 microns or 300 microns.

In this example, the adhesive layer 202 b and layers of the foldabledisplay stack 111 form the resonator 250 a, which is bounded by theglass layer 203 and the display stiffener 113. In some examples, thethickness of the resonator 250 a may correspond to a multiple N of ahalf wavelength at a peak frequency of an ultrasonic frequency range ofultrasonic waves transmitted by the ultrasonic sensor stack 105 a, whereN is an integer greater than or equal to 1. In some suchimplementations, the resonator 250 a may cause a local maximum within anultrasonic frequency range. According to some examples, the localmaximum may correspond to a frequency in the range from 5 MHz to 15 MHz,or from 1 MHz to 20 MHz. According to some such implementations, afrequency range that includes the local ultrasonic wave transmissionmaximum caused by the resonator 250 a may correspond with a frequencyrange that includes a local ultrasonic wave transmission maximum causedby the resonator 250 c.

According to this example, the foldable display stack 111 also includesthe resonator 250 c, which is formed by the one or more screen protectorlayers 211 and the OCA 204 a, and is bounded by the glass layer 203:here, the glass layer 203 has a higher acoustic impedance than that ofthe one or more screen protector layers 211 or the OCA 204 a. In someexamples, the thickness of the resonator 250 c may correspond to amultiple N of a quarter wavelength at a peak frequency of an ultrasonicfrequency range of ultrasonic waves transmitted by the ultrasonic sensorstack 105 a, where N is an integer greater than or equal to 1. In somesuch implementations, the resonator 250 c may cause a local maximumwithin the ultrasonic frequency range. According to some examples, thelocal maximum may correspond to a frequency in the range from 5 MHz to15 MHz, or from 1 MHz to 20 MHz. According to some such implementations,a frequency range that includes the local ultrasonic wave transmissionmaximum caused by the resonator 250 a may correspond with a frequencyrange that includes a local ultrasonic wave transmission maximum causedby the resonator 250 c.

In this implementation, the glass layer 203 forms a resonator 250 b.According to some examples, the frequencies for maximum transmission ofultrasonic waves through the resonator 250 b may be outside of (e.g.,below) the frequency range that includes the local maximum caused by theresonator 250 a.

In this example, the resonator 250 e includes the display stiffener 113,the transmission enhancement layer 103 and the TFT layer 102, as well asthe adhesive layers 202 a and 202 c. Although the resonator 250 d formedby the display stiffener 113 alone causes a very low transmission ofultrasonic waves in the frequency range from 5 MHz to 15 MHz, in thisexample the resonator 250 e causes at least one local ultrasonic wavetransmission maximum in the frequency range from 5 MHz to 15 MHz, orfrom 1 MHz to 20 MHz.

The resonator 250 e is one example of what may be referred to herein asa “transmission enhancement resonator,” which is configured to enhancethe ultrasonic waves transmitted by the ultrasonic sensor stack in atleast a first ultrasonic frequency range (for example, the frequencyband 301 of FIG. 3 or the frequency band 501 of FIG. 5 ). According tothe example shown in FIG. 4 , the transmission enhancement layer 103,the display stiffener 113, at least a portion of the ultrasonic sensorstack 105 a (e.g., the TFT layer 102) forming the transmissionenhancement resonator 250 e. The transmission enhancement resonator 250e may, in some instances, be configured to cause ultrasonic wavestransmitted by the ultrasonic sensor stack 105 a to have a localamplitude maximum (which may be referred to herein as a “transmissionenhancement resonator-induced local maximum”) in the first ultrasonicfrequency range. Such a local maximum is one example of what is referredto herein as “enhancing” the ultrasonic waves transmitted by theultrasonic sensor stack in the first ultrasonic frequency range. In someexamples, the transmission enhancement resonator 250 e may “enhance” theultrasonic waves in an ultrasonic frequency range by allowing theultrasonic waves in that frequency range to pass at a relatively higheramplitude, as compared to the amplitude of ultrasonic waves in thatfrequency range in an apparatus that does not include the transmissionenhancement resonator 250 e. In some such implementations, thetransmission enhancement resonator 250 e may “enhance” the ultrasonicwaves in an ultrasonic frequency range by acting as a bandpass filterfor ultrasonic waves in the ultrasonic frequency range. According tosome examples, the transmission enhancement resonator 250 e may“enhance” the ultrasonic waves in an ultrasonic frequency range byreducing the acoustic impedance of the display stiffener 113 alone (inother words, of the display stiffener 113 in an apparatus lacking thetransmission enhancement resonator 250 e, such as that shown in FIG. 2 )to ultrasonic waves in the ultrasonic frequency range. The combinationof the transmission enhancement layer 103 and the display stiffener 113(and, in some examples, at least a portion of the ultrasonic sensorstack 105 a) may allow the ultrasonic waves in that frequency range topass at a relatively higher amplitude, as compared to the amplitude ofultrasonic waves in that frequency range passed by an apparatus thatdoes not include the transmission enhancement resonator 250 e, and/ormay cause a local amplitude maximum in that frequency range.

The specific range of frequencies (and/or the frequency corresponding toa local ultrasonic wave transmission maximum) may, in someimplementations, be selected to match a high-transmission frequencyrange of one or more resonators of the foldable display stack 111. Insome such examples, the thickness of the resonator 250 e may correspondto a multiple N of a half wavelength at a peak frequency of anultrasonic frequency range of ultrasonic waves transmitted by theultrasonic sensor stack 105 a, where N is an integer greater than orequal to 1. According to some such implementations, a frequency rangethat includes the local ultrasonic wave transmission maximum caused bythe resonator 250 e may correspond with a frequency range that includesa local ultrasonic wave transmission maximum caused by the resonator 250a and/or a frequency range that includes a local ultrasonic wavetransmission maximum caused by the resonator 250 c.

FIG. 5 shows examples of ultrasonic transmission versus ultrasonicfrequency graphs corresponding to examples of the resonators shown inFIG. 4 . In these examples, graph 505 a indicates the ultrasonictransmission amplitudes of the resonator 250 a at various frequencies,graph 505 b indicates the ultrasonic transmission amplitudes of theresonator 250 b at various frequencies, graph 505 c indicates theultrasonic transmission amplitudes of the resonator 250 c at variousfrequencies and graph 505 e indicates the ultrasonic transmissionamplitudes of the resonator 250 e at various frequencies. However, theseultrasonic transmission amplitudes are merely specific examples, basedon the thicknesses and materials used in a particular foldable displaystack 111.

In these examples, the frequency band 501 between frequencies F1 and F2is a desirable frequency band for ultrasonic imaging relating toauthentication. In some examples, the frequency band 501 may be suitablefor fingerprint imaging. In some implementations, the frequency band 501may be suitable for sub-epidermal imaging. In some instances, thefrequency band 501 may include frequencies in the range from 5 MHz to 15MHz, or from 1 MHz to 20 MHz. In one example, the frequency band 501 maybe in the range of 6 MHz to 8 MHz. In another example, the frequencyband 501 may be in the range of 11 MHz to 14 MHz.

According to the examples shown in FIG. 5 , the resonators 250 a and 250c cause a local ultrasonic transmission amplitude maximum within thefrequency band 501. In these examples, the peak frequency of the localultrasonic transmission amplitude maximum caused by the resonator 250 ais lower than the peak frequency of the local ultrasonic transmissionamplitude maximum caused by the resonator 250 c. In this instance, graph505 b indicates no local ultrasonic transmission amplitude maximumcaused by the resonator 250 b within the frequency band 501. However,the level of ultrasonic transmission caused by the resonator 250 b isgreater for frequencies within the frequency band 501 than forfrequencies that are higher than those within the frequency band 501.

Unlike the above-described resonator 250 d—corresponding to the displaystiffener 113 shown in FIG. 2 alone—the resonator 250 e causes a localultrasonic transmission amplitude peak within the frequency band 501. Asnoted above, the resonator 250 e is one example of what may be referredto herein as a “transmission enhancement resonator,” which is configuredto enhance the ultrasonic waves transmitted by the ultrasonic sensorstack in at least a first ultrasonic frequency range (in this example,the frequency band 501). The local ultrasonic transmission amplitudepeak within the frequency band 501 is one example of what is referred toherein as “enhancing” the ultrasonic waves transmitted by the ultrasonicsensor stack in the first ultrasonic frequency range. In some examples,the transmission enhancement resonator 250 e may “enhance” theultrasonic waves in an ultrasonic frequency range by allowing theultrasonic waves in that frequency range to pass at a relatively higheramplitude, as compared to the amplitude of ultrasonic waves in thatfrequency range in an apparatus that does not include the transmissionenhancement resonator 250 e. In some such implementations, thetransmission enhancement resonator 250 e may “enhance” the ultrasonicwaves in an ultrasonic frequency range by acting as a bandpass filterfor ultrasonic waves in the ultrasonic frequency range.

FIG. 6 shows examples of a foldable display stack and an ultrasonicsensor stack. As with other disclosed implementations, the types, numberand arrangement of elements shown in FIG. 6 are merely examples. Otherimplementations may include different types, numbers and/or arrangementsof elements. Moreover, the elements shown in FIG. 6 are not drawn toscale.

In this example the apparatus 100 b is similar to the apparatus 100 ashown in FIG. 4 , because in this implementation the apparatus 100 bincludes an instance of the transmission enhancement layer 103 that isdescribed above. As with the example of the apparatus 100 a that isdescribed above with reference to FIG. 4 , the apparatus 100 b alsoincludes instances of the ultrasonic sensor stack 105 and of thefoldable display stack 111 of FIG. 1 . As in the example that isdescribed above with reference to FIG. 4 , in this example thetransmission enhancement layer 103 is attached to the foldable displaystack 111 via an adhesive layer 202 a and the ultrasonic sensor stack105 b is attached to the transmission enhancement layer 103 via anadhesive layer 202 c. In some examples, the adhesive layers 202 a and/or202 c may be, or may include, a thin pressure-sensitive adhesive (PSA).In some instances, the adhesive layers 202 a and/or 202 c may be in therange of 1 micron to 10 microns.

According to this example, the ultrasonic sensor stack 105 b includes aTFT layer 102, an ultrasonic transceiver layer 101, an electrode layer210 and a passivation layer 212. However, unlike the implementation thatis described above with reference to FIG. 4 , in this example theultrasonic transceiver layer 101, the electrode layer 210 and thepassivation layer 212 reside between the TFT layer 102 and thetransmission enhancement layer 103.

In this example, the foldable display stack 111 includes the same layersthat are described above with reference to FIGS. 2 and 4 . Otherimplementations may provide other foldable display stack layers,foldable display stack layers having different materials and/orthicknesses, etc.

In this example, the adhesive layer 202 b and layers of the foldabledisplay stack 111 form the resonator 250 a, which is bounded by theglass layer 203 and the display stiffener 113. In some examples, thethickness of the resonator 250 a may correspond to a multiple N of ahalf wavelength at a peak frequency of an ultrasonic frequency range ofultrasonic waves transmitted by the ultrasonic sensor stack 105 b, whereN is an integer greater than or equal to 1. In some suchimplementations, the resonator 250 a may cause a local maximum within anultrasonic frequency range. According to some examples, the localmaximum may correspond to a frequency in the range from 5 MHz to 15 MHz,or from 1 MHz to 20 MHz. According to some such implementations, afrequency range that includes the local ultrasonic wave transmissionmaximum caused by the resonator 250 a may correspond with a frequencyrange that includes a local ultrasonic wave transmission maximum causedby the resonator 250 c.

According to this example, the foldable display stack 111 also includesthe resonator 250 c, which is formed by the one or more screen protectorlayers 211 and the OCA 204 a, and is bounded by the glass layer 203:here, the glass layer 203 has a higher acoustic impedance than that ofthe one or more screen protector layers 211 or the OCA 204 a. In someexamples, the thickness of the resonator 250 c may correspond to amultiple N of a quarter wavelength at a peak frequency of an ultrasonicfrequency range of ultrasonic waves transmitted by the ultrasonic sensorstack 105 b, where N is an integer greater than or equal to 1. In somesuch implementations, the resonator 250 c may cause a local maximumwithin the ultrasonic frequency range. According to some examples, thelocal maximum may correspond to a frequency in the range from 5 MHz to15 MHz, or from 1 MHz to 20 MHz. According to some such implementations,a frequency range that includes the local ultrasonic wave transmissionmaximum caused by the resonator 250 a may correspond with a frequencyrange that includes a local ultrasonic wave transmission maximum causedby the resonator 250 c.

In this implementation, the glass layer 203 forms a resonator 250 b.According to some examples, the frequencies for maximum transmission ofultrasonic waves through the resonator 250 b may be outside of (e.g.,below) the frequency range that includes the local maximum caused by theresonator 250 a.

The resonator 250 f is another example of what may be referred to hereinas a “transmission enhancement resonator,” which is configured toenhance the ultrasonic waves transmitted by the ultrasonic sensor stackin at least a first ultrasonic frequency range (for example, thefrequency band 301 of FIG. 3 or the frequency band 501 of FIG. 5 ). Inthis example, the resonator 250 f includes the display stiffener 113,the transmission enhancement layer 103 and the adhesive layer 202 a.

According to this implementation and unlike the implementation that isdescribed above with reference to FIG. 4 , the transmission enhancementresonator does not include the TFT layer 102 in this example. This isbecause the ultrasonic transceiver layer 101, the electrode layer 210and the passivation layer 212, all of which generally have a loweracoustic impedance than those of the display stiffener 113 and thetransmission enhancement layer 103, reside between the TFT layer 102 andthe transmission enhancement layer 103 in this example. In some suchexamples, the thickness of the resonator 250 f may correspond to amultiple N of a half wavelength at a peak frequency of an ultrasonicfrequency range of ultrasonic waves transmitted by the ultrasonic sensorstack 105 b, where N is an integer greater than or equal to 1. In someexamples, the thickness of the resonator 250 f may be in a range from50-150 microns. Although the resonator 250 d formed by the displaystiffener 113 causes a very low transmission of ultrasonic waves in thefrequency range from 5 MHz to 15 MHz, in this example the resonator 250f causes at least one local ultrasonic wave transmission maximum in thefrequency range from 5 MHz to 15 MHz, or from 1 MHz to 20 MHz.

The specific range of frequencies (and/or the frequency corresponding toa local ultrasonic wave transmission maximum) may, in someimplementations, be selected to match a high-transmission frequencyrange of one or more resonators of the foldable display stack 111.According to some such implementations, a frequency range that includesthe local ultrasonic wave transmission maximum caused by the resonator250 f may correspond with a frequency range that includes a localultrasonic wave transmission maximum caused by the resonator 250 aand/or a frequency range that includes a local ultrasonic wavetransmission maximum caused by the resonator 250 c.

FIG. 7 shows example components of an apparatus according to somedisclosed implementations. As with other disclosed implementations, thetypes, number and arrangement of elements, as well as the dimensions ofelements, are merely examples. According to this example, the apparatus100 c is configured to perform at least some of the methods disclosedherein. According to this implementation, the ultrasonic sensor stack105 a includes a ultrasonic transceiver layer 101, an electrode layer210 on one side of the ultrasonic transceiver layer 101 and an array ofsensor pixels 706 on a second and opposing side of the ultrasonictransceiver layer 101. In this implementation, the ultrasonictransceiver layer 101 includes one or more piezoelectric polymers. Inother implementations, the ultrasonic transceiver layer 101 may includeother types of piezoelectric materials.

According to this example, the electrode layer 210 resides between apassivation layer 212 and the ultrasonic transceiver layer 101. In someexamples, the passivation layer 212 may include an adhesive, such as anepoxy film, a polymer layer (such as a polyethylene terephthalate (PET)layer), etc.

In this example the TFT layer 102 includes a TFT substrate and circuitryfor the array of sensor pixels 706. The TFT layer 102 may be a type ofmetal-oxide-semiconductor field-effect transistor (MOSFET) made bydepositing thin films of an active semiconductor layer as well as adielectric layer and metallic contacts over a TFT substrate. In someexamples, the TFT substrate may be a non-conductive material such asglass.

In this example, the apparatus 100 c includes a foldable display stack111, which includes a display stiffener 113 and display stack layers 115in this instance. According to this example, the display stack layers115 form one or more display stack resonators. In some instances, theone or more display stack resonators may be, or may include, theresonators 250 a, 250 b and/or 250 c that are described above.

According to some examples, a transmission enhancement resonator thatincludes the display stiffener 113 and the transmission enhancementlayer 103 may be configured to enhance the ultrasonic waves transmittedby the ultrasonic sensor stack 105 a in at least one ultrasonicfrequency range. In some implementations, the transmission enhancementresonator also may include the TFT layer 102. In some examples, thetransmission enhancement resonator may cause a local maximum ofultrasonic wave transmission in at least one ultrasonic frequency range.The at least one ultrasonic frequency range may, in some instances, be afrequency range in which one or more display stack resonators cause alocal maximum of ultrasonic wave transmission.

According to this implementation, the TFT layer 102, the array of sensorpixels 706 and the electrode are electrically coupled to at least aportion of the control system 106 and one side of the ultrasonictransceiver layer 101 via a portion of the interface system 107, whichincludes electrically conducting material and a flexible printed circuit(FPC) in this instance.

In this example, the apparatus 100 c is configured to perform at leastsome of the methods disclosed herein. In this example, the controlsystem 106 is configured to control the ultrasonic sensor system totransmit one or more ultrasonic waves 713. According to this example,the ultrasonic waves 713 are transmitted through the TFT layer 102, thetransmission enhancement layer 103, the display stiffener 113 and thedisplay stack layers 115. According to this example, reflections 714 ofthe ultrasonic waves 713 are caused by acoustic impedance contrast at(or near) the interface 715 between the outer surface of the cover 718and whatever is in contact with the outer surface, which may be air orthe surface of a target object, such as the ridges and valleys of afingerprint, etc. (As used herein, the term “finger” may refer to anydigit, including a thumb. Accordingly, a thumbprint will be considered atype of “fingerprint.”)

According to some examples, reflections 714 of the ultrasonic wave(s)713 may be detected by the array of sensor pixels 706. Correspondingultrasonic signals may be provided to the control system 106. In somesuch implementations, ultrasonic signals that are used by the controlsystem 106 for fingerprint-based authentication may be based onreflections 714 from a cover/finger interface that are detected by thearray of sensor pixels 706. In some implementations, reflections 714corresponding to a cover/air interface may be detected by the electrodelayer 210 and corresponding background ultrasonic signals may beprovided to the control system 106.

FIG. 8 is a flow diagram that provides examples of operations accordingto some disclosed methods. The blocks of FIG. 8 may, for example, beperformed by the apparatus 100 of FIG. 1 (e.g., by the control system106 and the ultrasonic sensor stack 105), or by a similar apparatus thatincludes a frequency-differentiating layer. As with other methodsdisclosed herein, the method outlined in FIG. 8 may include more orfewer blocks than indicated. Moreover, the blocks of methods disclosedherein are not necessarily performed in the order indicated. In someinstances, one or more blocks may be performed concurrently.

In this example, block 805 involves controlling, via a control system(e.g., via the control system 106) an ultrasonic transceiver layer of anultrasonic sensor system (e.g., the ultrasonic transceiver layer 101) totransmit ultrasonic waves (e.g., the ultrasonic waves 713 shown in FIG.7 ) through at least a first resonator configured for causing a firstlocal maximum of ultrasonic wave transmission in a first ultrasonicfrequency range and a second resonator configured for causing a secondlocal maximum of ultrasonic wave transmission in the first ultrasonicfrequency range. According to this example, the first resonator includesa display stiffener layer (such as the display stiffener 113) and atransmission enhancement layer (such as the transmission enhancementlayer 103). In some examples, the first resonator may include a TFTlayer.

The first resonator is an example of what may be referred to herein as atransmission enhancement resonator. In some examples, the firstresonator may have a thickness corresponding to a multiple of a halfwavelength of a shear wave or a longitudinal wave having a frequency inthe first ultrasonic frequency range. In some instances, the firstultrasonic frequency range may be the range from 5 MHz to 15 MHz. Insome examples, the first ultrasonic frequency range may be the rangefrom 1 MHz to 20 MHz.

In this implementation, the second resonator includes one or moredisplay stack layers. In some instances, the second resonator may be theresonator 250 a or the resonator 250 c described above.

According to this implementation, block 810 involves receiving, by thecontrol system and from the ultrasonic sensor system, ultrasonic sensorsignals corresponding to reflections of transmitted ultrasonic wavesfrom a portion of a target object positioned on an outer surface of anapparatus that includes the ultrasonic sensor system. According to someexamples, the ultrasonic sensor signals may correspond to reflectionsfrom an interior of the portion of the target object. If the targetobject is a finger, the first signals may correspond to reflections ofthe first ultrasonic wave(s) from a subsurface of the finger, e.g., ofreflections from one or more sub-epidermal features. Alternatively, oradditionally, the ultrasonic sensor signals may correspond toreflections of the transmitted ultrasonic waves from a surface of theportion of the target object. If the target object is a finger, theultrasonic sensor signals may correspond to reflections of the secondultrasonic wave(s) from a surface of the finger, e.g., from ridges andvalleys of a fingerprint.

According to this implementation, block 815 involves performing, by thecontrol system, an authentication process that is based, at least inpart, on the ultrasonic sensor signals. In some implementations, method800 may involve controlling access to the apparatus, or to anotherdevice, based at least in part on the authentication process.

According to some implementations, block 815 may involve obtainingfingerprint data based on portions of the ultrasonic sensor signalsreceived within a time interval corresponding with fingerprints. Thetime interval may, for example, be measured relative to a time at whichthe ultrasonic waves were transmitted. Obtaining the fingerprint datamay, for example, involve extracting target object features from theultrasonic sensor signals. The target object features may, for example,comprise fingerprint features. According to some examples, thefingerprint features may include fingerprint minutiae, keypoints and/orsweat pores. In some examples, the fingerprint features may includeridge ending information, ridge bifurcation information, short ridgeinformation, ridge flow information, island information, spurinformation, delta information, core information, etc.

In some examples, block 815 may involve comparing the fingerprintfeatures with fingerprint features of an authorized user. Thefingerprint features of the authorized user may, for example, have beenreceived during a previous enrollment process.

In some implementations, block 815 may involve extracting sub-epidermalfeatures from the ultrasonic sensor signals. Sub-epidermal features ofthe authorized user may, for example, have been received during aprevious enrollment process. According to some implementations, theauthentication process may involve comparing sub-epidermal featuresextracted from the ultrasonic sensor signals with sub-epidermal featuresof the authorized user.

In some such implementations, the sub-epidermal features may includesub-epidermal layer information corresponding to reflections of theultrasonic waves received from the portion of the target object within atime interval corresponding with a sub-epidermal region. Thesub-epidermal features may, for example, include dermis layerinformation corresponding to reflections of the second ultrasonic wavereceived from the portion of the target object. The dermis layerinformation may have been obtained within a time interval correspondingwith a dermis layer. The authentication process may be based, at leastin part, on the dermis layer information. Alternatively, oradditionally, the sub-epidermal features may include informationregarding other sub-epidermal layers, such as the papillary layer, thereticular layer, the subcutis, etc., any blood vessels, lymph vessels,sweat glands, hair follicles, hair papilla, fat lobules, etc., that maybe present within such tissue layers, muscle tissue, bone material, etc.

FIG. 9 representationally depicts aspects of a 4×4 pixel array of sensorpixels for an ultrasonic sensor system. Each pixel 934 may be, forexample, associated with a local region of piezoelectric sensor material(PSM), a peak detection diode (D1) and a readout transistor (M3); manyor all of these elements may be formed on or in a substrate to form thepixel circuit 936. In practice, the local region of piezoelectric sensormaterial of each pixel 934 may transduce received ultrasonic energy intoelectrical charges. The peak detection diode D1 may register the maximumamount of charge detected by the local region of piezoelectric sensormaterial PSM. Each row of the pixel array 935 may then be scanned, e.g.,through a row select mechanism, a gate driver, or a shift register, andthe readout transistor M3 for each column may be triggered to allow themagnitude of the peak charge for each pixel 934 to be read by additionalcircuitry, e.g., a multiplexer and an A/D converter. The pixel circuit936 may include one or more TFTs to allow gating, addressing, andresetting of the pixel 934.

Each pixel circuit 936 may provide information about a small portion ofthe object detected by the ultrasonic sensor system. While, forconvenience of illustration, the example shown in FIG. 9 is of arelatively coarse resolution, ultrasonic sensors having a resolution onthe order of 500 pixels per inch or higher may be configured with anappropriately scaled structure. The detection area of the ultrasonicsensor system may be selected depending on the intended object ofdetection. For example, the detection area may range from about 5 mm×5mm for a single finger to about 3 inches×3 inches for four fingers.Smaller and larger areas, including square, rectangular andnon-rectangular geometries, may be used as appropriate for the targetobject.

Implementation examples are described in the following numbered clauses:

1. An apparatus, comprising: an ultrasonic sensor stack; a foldabledisplay stack, comprising a display stiffener and display stack layersconfigured to cause ultrasonic waves transmitted by the ultrasonicsensor stack to have one or more display stack-induced local amplitudemaxima in a first ultrasonic frequency range; and a transmissionenhancement layer, wherein the display stiffener and the transmissionenhancement layer comprise a transmission enhancement resonatorconfigured to cause the ultrasonic waves transmitted by the ultrasonicsensor stack to have a transmission enhancement resonator-induced localamplitude maximum in the first ultrasonic frequency range.

2. The apparatus of clause 1, wherein the transmission enhancementresonator has a thickness corresponding to a multiple of a halfwavelength of a shear wave or a longitudinal wave having a frequency inthe first ultrasonic frequency range.

3. The apparatus of clause 1 or clause 2, wherein the display stacklayers include one or more display stack resonators and wherein the oneor more display stack resonators are configured to cause the one or moredisplay stack-induced local amplitude maxima.

4. The apparatus of clause 3, wherein the one or more display stackresonators includes a first resonator bounded by the display stiffenerand a glass layer of the foldable display stack.

5. The apparatus of clause 4, wherein the first resonator includes aplurality of layers of an organic light-emitting diode display.

6. The apparatus of clause 4 or clause 5, wherein the one or moredisplay stack resonators includes a second resonator bounded by theglass layer and an outer surface of the foldable display stack.

7. The apparatus of any one of clauses 1-6, wherein the transmissionenhancement resonator-induced local amplitude maximum corresponds to afrequency in a range from 5 MHz to 15 MHz.

8. The apparatus of any one of clauses 1-7, further comprising a controlsystem configured to cause the ultrasonic sensor stack to transmit firstultrasonic waves in the first ultrasonic frequency range and to performan authentication process based, at least in part, on ultrasonic sensorsignals corresponding to reflections of the first ultrasonic waves.

9. The apparatus of any one of clauses 1-8, wherein the transmissionenhancement layer resides between the ultrasonic sensor stack and thedisplay stiffener.

10. The apparatus of any one of clauses 1-9, wherein the transmissionenhancement layer has a thickness of less than a quarter wavelength of ashear wave or a longitudinal wave having a frequency in the firstultrasonic frequency range.

11. The apparatus of any one of clauses 1-10, wherein the transmissionenhancement layer comprises an aluminum layer having an aluminum layerthickness in a range from 50 microns to 100 microns, a copper layerhaving a copper layer thickness in a range from 25 microns to 50 micronsor a stainless steel layer having a stainless steel layer thickness inthe range from 25 microns to 50 microns.

12. The apparatus of any one of clauses 1-11, further comprising a firstadhesive layer residing between the transmission enhancement layer andthe display stiffener and a second adhesive layer residing between thetransmission enhancement layer and the ultrasonic sensor stack.

13. The apparatus of clause 12, wherein at least one of the firstadhesive layer or the second adhesive layer has a thickness in a rangefrom 3 microns to 10 microns.

14. The apparatus of any one of clauses 1-13, wherein the ultrasonicsensor stack includes a thin-film transistor (TFT) substrate and whereinthe transmission enhancement resonator includes the TFT substrate.

15. The apparatus of clause 14, wherein the TFT substrate has athickness of less than a quarter wavelength of a shear wave or alongitudinal wave having a frequency in the first ultrasonic frequencyrange.

16. The apparatus of clause 14 or clause 15, wherein the TFT substratehas a thickness in a range from 50 microns to 200 microns.

17. The apparatus of any one of clauses 14-16, wherein the TFT substratecomprises glass.

18. The apparatus of any one of clauses 14-17, wherein: the TFTsubstrate has a first acoustic impedance value; the display stiffenerhas a second acoustic impedance value that is greater than the firstacoustic impedance value; and the transmission enhancement layer has athird acoustic impedance value that is greater than the first acousticimpedance value.

19. The apparatus of any one of clauses 1-18, wherein the displaystiffener comprises one or more of a metal layer or a non-metal layerhaving an acoustic impedance of 10 MRayls or more.

20. The apparatus of any one of clauses 1-19, wherein the displaystiffener has a thickness in a range from 30 microns to 300 microns.

21. The apparatus of any one of clauses 1-20, wherein the displaystiffener has a thickness corresponding to a multiple of a halfwavelength of a shear wave or a longitudinal wave having a frequency ina second ultrasonic frequency range that is higher than the firstultrasonic frequency range.

22. The apparatus of any one of clauses 1-21, wherein the apparatus is amobile device that includes the ultrasonic sensor stack, the foldabledisplay stack and the transmission enhancement layer.

23. The apparatus of any one of clauses 1-22, wherein the transmissionenhancement layer includes at least a portion of the ultrasonic sensorstack.

Further implementation examples are described in the following numberedclauses:

24. An apparatus, comprising: ultrasonic sensor means; a foldabledisplay stack, comprising a display stiffener and one or more displaystack resonators configured to enhance ultrasonic waves transmitted bythe ultrasonic sensor means in a first ultrasonic frequency range; and atransmission enhancement layer, the display stiffener, the transmissionenhancement layer and at least a portion of the ultrasonic sensor meansforming a transmission enhancement resonator configured to enhance theultrasonic waves transmitted by the ultrasonic sensor means in the firstultrasonic frequency range.

25. The apparatus of clause 24, wherein the transmission enhancementresonator has a thickness corresponding to a multiple of a halfwavelength of a shear wave or a longitudinal wave having a frequency inthe first ultrasonic frequency range.

26. The apparatus of clause 24, wherein enhancing an ultrasonic wave inthe first ultrasonic frequency range comprises causing a local maximumof the ultrasonic wave in the first ultrasonic frequency range.

Further implementation examples are described in the following numberedclauses:

27. A method, comprising: controlling, via a control system, anultrasonic transceiver layer of an ultrasonic sensor system to transmitultrasonic waves through at least a first resonator configured forcausing a first local maximum of ultrasonic wave transmission in a firstultrasonic frequency range and a second resonator configured for causinga second local maximum of ultrasonic wave transmission in the firstultrasonic frequency range, the first resonator including a displaystiffener layer and a transmission enhancement layer, the secondresonator including one or more display stack layers; receiving, by thecontrol system and from the ultrasonic sensor system, ultrasonic sensorsignals corresponding to reflections of transmitted ultrasonic wavesfrom a portion of a target object positioned on an outer surface of anapparatus that includes the ultrasonic sensor system; and performing, bythe control system, an authentication process based, at least in part,on the ultrasonic sensor signals.

28. The method of clause 27, wherein the local maximum of ultrasonicwave transmission corresponds to a frequency in a range from 5 MHz to 15MHz.

29. The method of clause 27, wherein the authentication process involvesextracting target object features from the ultrasonic sensor signals.

30. The method of clause 29, wherein the target object features includeat least one of fingerprint features or sub-epidermal features.

31. The method of clause 27, further comprising controlling access tothe apparatus based, at least in part, on the authentication process.

As used herein, a phrase referring to “at least one of” a list of itemsrefers to any combination of those items, including single members. Asan example, “at least one of: a, b, or c” is intended to cover: a, b, c,a-b, a-c, b-c, and a-b-c.

The various illustrative logics, logical blocks, modules, circuits andalgorithm processes described in connection with the implementationsdisclosed herein may be implemented as electronic hardware, computersoftware, or combinations of both. The interchangeability of hardwareand software has been described generally, in terms of functionality,and illustrated in the various illustrative components, blocks, modules,circuits and processes described above. Whether such functionality isimplemented in hardware or software depends upon the particularapplication and design constraints imposed on the overall system.

The hardware and data processing apparatus used to implement the variousillustrative logics, logical blocks, modules and circuits described inconnection with the aspects disclosed herein may be implemented orperformed with a general purpose single- or multi-chip processor, adigital signal processor (DSP), an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA) or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described herein. A general purpose processor may be amicroprocessor, or, any conventional processor, controller,microcontroller, or state machine. A processor also may be implementedas a combination of computing devices, e.g., a combination of a DSP anda microprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration. In some implementations, particular processes and methodsmay be performed by circuitry that is specific to a given function.

In one or more aspects, the functions described may be implemented inhardware, digital electronic circuitry, computer software, firmware,including the structures disclosed in this specification and theirstructural equivalents thereof, or in any combination thereof.Implementations of the subject matter described in this specificationalso may be implemented as one or more computer programs, i.e., one ormore modules of computer program instructions, encoded on a computerstorage media for execution by, or to control the operation of, dataprocessing apparatus.

If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium, such as a non-transitory medium. The processesof a method or algorithm disclosed herein may be implemented in aprocessor-executable software module which may reside on acomputer-readable medium. Computer-readable media include both computerstorage media and communication media including any medium that may beenabled to transfer a computer program from one place to another.Storage media may be any available media that may be accessed by acomputer. By way of example, and not limitation, non-transitory mediamay include RAM, ROM, EEPROM, CD-ROM or other optical disk storage,magnetic disk storage or other magnetic storage devices, or any othermedium that may be used to store desired program code in the form ofinstructions or data structures and that may be accessed by a computer.Also, any connection may be properly termed a computer-readable medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk, and Blu-raydisc where disks usually reproduce data magnetically, while discsreproduce data optically with lasers. Combinations of the above shouldalso be included within the scope of computer-readable media.Additionally, the operations of a method or algorithm may reside as oneor any combination or set of codes and instructions on a machinereadable medium and computer-readable medium, which may be incorporatedinto a computer program product.

Various modifications to the implementations described in thisdisclosure may be readily apparent to those having ordinary skill in theart, and the generic principles defined herein may be applied to otherimplementations without departing from the spirit or scope of thisdisclosure. Thus, the disclosure is not intended to be limited to theimplementations shown herein, but is to be accorded the widest scopeconsistent with the claims, the principles and the novel featuresdisclosed herein. The word “exemplary” is used exclusively herein, if atall, to mean “serving as an example, instance, or illustration.” Anyimplementation described herein as “exemplary” is not necessarily to beconstrued as preferred or advantageous over other implementations.

Certain features that are described in this specification in the contextof separate implementations also may be implemented in combination in asingle implementation. Conversely, various features that are describedin the context of a single implementation also may be implemented inmultiple implementations separately or in any suitable subcombination.Moreover, although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination may in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemsmay generally be integrated together in a single software product orpackaged into multiple software products. Additionally, otherimplementations are within the scope of the following claims. In somecases, the actions recited in the claims may be performed in a differentorder and still achieve desirable results.

It will be understood that unless features in any of the particulardescribed implementations are expressly identified as incompatible withone another or the surrounding context implies that they are mutuallyexclusive and not readily combinable in a complementary and/orsupportive sense, the totality of this disclosure contemplates andenvisions that specific features of those complementary implementationsmay be selectively combined to provide one or more comprehensive, butslightly different, technical solutions. It will therefore be furtherappreciated that the above description has been given by way of exampleonly and that modifications in detail may be made within the scope ofthis disclosure.

The invention claimed is:
 1. An apparatus, comprising: an ultrasonicsensor stack; a foldable display stack, comprising: a display stiffener;and display stack layers configured to cause ultrasonic wavestransmitted by the ultrasonic sensor stack to have one or more displaystack-induced local amplitude maxima in a first ultrasonic frequencyrange; and a transmission enhancement layer, wherein the displaystiffener and the transmission enhancement layer comprise a transmissionenhancement resonator configured to cause the ultrasonic wavestransmitted by the ultrasonic sensor stack to have a transmissionenhancement resonator-induced local amplitude maximum in the firstultrasonic frequency range.
 2. The apparatus of claim 1, wherein thetransmission enhancement resonator has a thickness corresponding to amultiple of a half wavelength of a shear wave or a longitudinal wavehaving a frequency in the first ultrasonic frequency range.
 3. Theapparatus of claim 1, wherein the display stack layers include one ormore display stack resonators and wherein the one or more display stackresonators are configured to cause the one or more display stack-inducedlocal amplitude maxima.
 4. The apparatus of claim 3, wherein the one ormore display stack resonators includes a first resonator bounded by thedisplay stiffener and a glass layer of the foldable display stack. 5.The apparatus of claim 4, wherein the first resonator includes aplurality of layers of an organic light-emitting diode display.
 6. Theapparatus of claim 4, wherein the one or more display stack resonatorsincludes a second resonator bounded by the glass layer and an outersurface of the foldable display stack.
 7. The apparatus of claim 1,wherein the transmission enhancement resonator-induced local amplitudemaximum corresponds to a frequency in a range from 5 MHz to 15 MHz. 8.The apparatus of claim 1, further comprising a control system configuredto cause the ultrasonic sensor stack to transmit first ultrasonic wavesin the first ultrasonic frequency range and to perform an authenticationprocess based, at least in part, on ultrasonic sensor signalscorresponding to reflections of the first ultrasonic waves.
 9. Theapparatus of claim 1, wherein the transmission enhancement layer residesbetween the ultrasonic sensor stack and the display stiffener.
 10. Theapparatus of claim 1, wherein the transmission enhancement layer has athickness of less than a quarter wavelength of a shear wave or alongitudinal wave having a frequency in the first ultrasonic frequencyrange.
 11. The apparatus of claim 1, wherein the transmissionenhancement layer comprises an aluminum layer having an aluminum layerthickness in a range from 50 microns to 100 microns, a copper layerhaving a copper layer thickness in a range from 25 microns to 50 micronsor a stainless steel layer having a stainless steel layer thickness inthe range from 25 microns to 50 microns.
 12. The apparatus of claim 1,further comprising a first adhesive layer residing between thetransmission enhancement layer and the display stiffener and a secondadhesive layer residing between the transmission enhancement layer andthe ultrasonic sensor stack.
 13. The apparatus of claim 12, wherein atleast one of the first adhesive layer or the second adhesive layer has athickness in a range from 3 microns to 10 microns.
 14. The apparatus ofclaim 1, wherein the ultrasonic sensor stack includes a thin-filmtransistor (TFT) substrate and wherein the transmission enhancementresonator includes the TFT substrate.
 15. The apparatus of claim 14,wherein the TFT substrate has a thickness of less than a quarterwavelength of a shear wave or a longitudinal wave having a frequency inthe first ultrasonic frequency range.
 16. The apparatus of claim 14,wherein the TFT substrate has a thickness in a range from 50 microns to200 microns.
 17. The apparatus of claim 14, wherein the TFT substratecomprises glass.
 18. The apparatus of claim 14, wherein: the TFTsubstrate has a first acoustic impedance value; the display stiffenerhas a second acoustic impedance value that is greater than the firstacoustic impedance value; and the transmission enhancement layer has athird acoustic impedance value that is greater than the first acousticimpedance value.
 19. The apparatus of claim 1, wherein the displaystiffener comprises one or more of a metal layer or a non-metal layerhaving an acoustic impedance of 10 MRayls or more.
 20. The apparatus ofclaim 1, wherein the display stiffener has a thickness in a range from30 microns to 300 microns.
 21. The apparatus of claim 1, wherein thedisplay stiffener has a thickness corresponding to a multiple of a halfwavelength of a shear wave or a longitudinal wave having a frequency ina second ultrasonic frequency range that is higher than the firstultrasonic frequency range.
 22. The apparatus of claim 1, wherein theapparatus is a mobile device that includes the ultrasonic sensor stack,the foldable display stack and the transmission enhancement layer. 23.The apparatus of claim 1, wherein the transmission enhancement layerincludes at least a portion of the ultrasonic sensor stack.
 24. Anapparatus, comprising: ultrasonic sensor means; a foldable displaystack, comprising: a display stiffener; and one or more display stackresonators configured to enhance ultrasonic waves transmitted by theultrasonic sensor means in a first ultrasonic frequency range; and atransmission enhancement layer, the display stiffener, the transmissionenhancement layer and at least a portion of the ultrasonic sensor meansforming a transmission enhancement resonator configured to enhance theultrasonic waves transmitted by the ultrasonic sensor means in the firstultrasonic frequency range.
 25. The apparatus of claim 24, wherein thetransmission enhancement resonator has a thickness corresponding to amultiple of a half wavelength of a shear wave or a longitudinal wavehaving a frequency in the first ultrasonic frequency range.
 26. Theapparatus of claim 24, wherein enhancing an ultrasonic wave in the firstultrasonic frequency range comprises causing a local maximum of theultrasonic wave in the first ultrasonic frequency range.
 27. A method,comprising: controlling, via a control system, an ultrasonic transceiverlayer of an ultrasonic sensor system to transmit ultrasonic wavesthrough at least a first resonator configured for causing a first localmaximum of ultrasonic wave transmission in a first ultrasonic frequencyrange and a second resonator configured for causing a second localmaximum of ultrasonic wave transmission in the first ultrasonicfrequency range, the first resonator including a display stiffener layerand a transmission enhancement layer, the second resonator including oneor more display stack layers; receiving, by the control system and fromthe ultrasonic sensor system, ultrasonic sensor signals corresponding toreflections of transmitted ultrasonic waves from a portion of a targetobject positioned on an outer surface of an apparatus that includes theultrasonic sensor system; and performing, by the control system, anauthentication process based, at least in part, on the ultrasonic sensorsignals.
 28. The method of claim 27, wherein the local maximum ofultrasonic wave transmission corresponds to a frequency in a range from5 MHz to 15 MHz.
 29. The method of claim 27, wherein the authenticationprocess involves extracting target object features from the ultrasonicsensor signals.
 30. The method of claim 29, wherein the target objectfeatures include at least one of fingerprint features or sub-epidermalfeatures.
 31. The method of claim 27, further comprising controllingaccess to the apparatus based, at least in part, on the authenticationprocess.